The mind’s tendency to adhere to the notion that one is not going to die is the source of all misfortune. The antidote to that attitude — recollection of death — is the source of all excellence. Therefore, you should not think, “This is a practice for those who do not have any profound teaching to meditate upon,” or “This is a genuine topic to meditate upon, but only briefly at the beginning [of my spiritual training]; it does not merit being made part of my regular practice.” Instead, you should undertake to meditate on this topic only after having developed the deep conviction that it is necessary at the beginning, middle, and end [of your entire career of spiritual training]. 

-- Lama Tsongkhapa